Biomimetic Reactive Oxygen Intermediates in Catalytic and Stoichiometric C−H Bond Activation

Abstract

Transition metal−reactive oxygen species, such as metal−oxo, −peroxo−hydroxo, and −aqua complexes, play significant roles in the numerous metalloenzymes and synthetic catalysts for small molecule activation. In biomimetic chemistry, many researchers have developed and examined synthetic model analogues that replicate the active sites of metalloenzymes to elucidate the electronic and geometric characteristics as well as reactivity of the metal−oxygen intermediates. In particular, metal−oxo and −hydroxo adducts have been suggested as crucial intermediates in biological systems. In this dissertation, we describe the catalytic and stoichiometric C−H bond activation by reactive oxygen intermediates and focus on modulating their reactivity by various synthetic methods. In chapter 1, it is revealed that the catalytic oxidation ability of a mononuclear copper(II)−alkylperoxo complex toward hydrocarbon substrates can be regulated by solvent and their polarity. In polar solvents, copper(II)−alkylperoxo complex activates weak C−H bonds, while copper(II)−alkylperoxo complex in nonpolar solvents can oxidize stronger C−H bonds up to cyclohexane. DFT calculations employing an implicit solvent model further supported the enhanced reactivity in various solvent systems and the different possible reactive intermediates. In chapter 2, controlling redox potential of a manganese(III)−bis(hydroxo) complex through protonation and the hydrogen-atom transfer reactivity is described. A series of mononuclear manganese(III)−hydroxo and −aqua complexes, were prepared and confirmed using various methods including X-ray crystallography. Thermodynamic analysis showed that protonation from hydroxo to aqua species resulted in increased redox potentials, while pKa values exhibited a reverse trend. Kinetic studies on electrophilic reactions using a variety of substrates revealed that manganese(III)–(bis)aqua complex, possessing the highest redox potential value, was found to undergo an aromatic C−H bond activation reaction under mild conditions. Keywords: Metalloenzyme, Metal−alkylperoxo complex, Metal−hydroxo intermediate, Metal−aqua species, Oxidative reaction, C−H bond activation.